It is known that red mud, which is produced as a waste product in the Bayer process for extracting aluminum hydroxide (ATH) from bauxite. In the following description red mud (RM) is understood to be the residue from the Bayer process which is produced in the extraction of ATH from bauxite.
Red mud (RM), which may to some extent be represented as bauxite minus ATH, is an extremely heterogeneous substance with regard to its chemical and mineralogical composition, its endothermic properties, its pH value, etc. The cause of the heterogeneity sometimes lies in the differing composition the bauxites used, but above all in whether the Bayer process operates by autoclave digestion or by tube digestion. In the autoclave process the digestion is carried out with 30 to 35% caustic soda solution at temperatures of 170-180° C., so that a pressure of 6 to 8 bars is established. The tube digestion process was developed in order to shorten the reaction time of 6 to 8 hours to less than 1 hour by increasing the temperature to 270° C. However, at this temperature a water vapor pressure of 60 bars is established at the end of the reactor. The higher temperatures of the tube digestion also influence the composition of the red mud. For example, in the iron hydroxide/oxide hydroxide system in the tube digestion process the balance is shifted almost completely towards hematite (Fe2O3). Because of the heterogeneity of the red mud (RM) the economically viable possibilities for use thereof is restricted, so that it must be predominantly disposed of as waste at disposal sites.
In WO 2012/126487 A1 a so-called “zero-halogen flame retardant” (OHFR) system, based upon modified rehydrated red mud (MR2S) is described, which is suitable as a cost-effective OHFR system for technical applications in the wire and cable field or for constructional and plastics processing applications. With the aid of the modified rehydrated red mud disclosed in WO 2012/126487 A1 a flame-retardant effect can be achieved in the temperature range from approximately 200° C.-350° C. The flame-retardant effect comes about due to the fact that the hydroxides and oxide hydroxides of aluminum and iron—such as for example gibbsite and boehmite or goethite—which are produced in the rehydration of the red mud decompose in oxides and water. Such products have applications for example in polymer systems such as PVC or EVA (PE). Products such as ATH or APP hitherto used in the market react between 180° C. and 220° C. and are regarded as low-temperature products. Between 220° C. and 340° C. products such as MDH and brucite are used which are regarded as high-temperature products. The flame retardants (MR2S) produced from RM by rehydration react between approximately 220° C. and 350° C. and thus according to the currently customary definition covers both the low-temperature and the high-temperature range.